The invention relates to a method of determining the position of a surface, which method comprises the following steps:
irradiating the surface with a radiation beam directed substantially perpendicularly to the surface and focusing the radiation beam by means of a focusing objective in a focus proximate to the surface; PA0 periodically displacing the focus formed into a direction perpendicular to the surface around an average position; PA0 detecting the radiation reflected by the surface on a radiation-sensitive detection system having a plurality of radiation-sensitive elements; and PA0 deriving a signal from the radiation distribution on the radiation-sensitive detection system, which signal is modulated due to said periodical displacement. PA0 irradiating the two surfaces with a radiation beam directed substantially perpendicularly to the surfaces and focusing the radiation beam by means of a focusing objective in a focus proximate to the surface; PA0 periodically displacing the focus formed into a direction substantially perpendicular to the surfaces around an average position at an amplitude which is larger than the depth of focus of the focusing objective; PA0 detecting the radiation reflected by the surfaces on a radiation-sensitive detection system having a plurality of radiation-sensitive elements; and PA0 deriving a signal from the radiation distribution on the radiation-sensitive detection system, which signal is modulated due to said periodical displacement; and PA0 displacing the surfaces and said average position with respect to each other until a first position at which a maximum amplitude of the modulated signal is obtained and subsequently displacing them once again to a second position at which a maximum amplitude of the signal is obtained.
The invention also relates to a device for performing this method.
Such a method and device are known from Netherlands Patent Application NL-A 7,608,561. This Application describes a method of correcting the focusing error which occurs when reading an optical record carrier by measuring the focusing error by means of an optical scanning device and by subsequently correcting this focusing error. The focus is moved up and down around the position of the surface of the record carrier with a small amplitude, which is smaller than the depth of focus. As soon as the focusing error is zero, the frequency of the output signal of the radiation-sensitive detection system is twice the frequency at which the focus is moved.
This method is satisfactory if the radiation beam is substantially focused on the surface when the measurement is started. However, if upon the start of the measurement the focus is at a distance from the surface which is much larger than the amplitude of the displacement, this method is less suitable. This problem occurs, for example when the distance between two (partly) reflecting surfaces must be measured, between which surfaces the distance is so small that the position of the second surface must be measured through the first surface.